The valve mechanism of an engine is responsible for providing fresh air for the engine cylinders, which is required for gasoline combustion and doing work, and for exhaust emission.
Camshafts are critical components of the valve mechanism. An engine is equipped with an admission camshaft and an emission camshaft, and each camshaft has a central shaft as well as a cam arranged round the central shaft. The central shaft is connected with the engine crankshaft via a drive chain or belt, and it rotates in synchronization with the cam. The cam of the admission camshaft is used to drive the admission valve of the engine for opening/closing while that of the emission camshaft is used to drive the emission valve of the engine for opening/closing.
In prior engines, the cam of an assembled camshaft is coupled with the central shaft by splines. Although splined coupling may ensure a precise cam angle position, it still has the following demerits:
Firstly, in order to form splines on the inner circumferential surface of the cam and the outer circumferential surface of the central shaft, the structure of the parts is made complicated. In addition, a special-purposed machine must be used to form the splines, and the parts are subject to carburization treatment to ensure a good part performance, resulting in complicated manufacturing process, long time consumption and high cost.
Secondly, the cam requires big axial dimensions, which is usually formed by means of casting process at a higher cost.
Thirdly, torque that can be transmitted by the splines is limited.
Besides the above, methods currently used to manufacture camshafts include: traditional one-piece casting method, forging method as well as assembled camshaft manufacturing method. Camshaft manufactured by casting method and forging method has a plural of demerits, for example, performance maximization design and utilization are impossible in various positions of the camshaft. This is because the performance requirements on the various parts including cam, journal, central shaft and drive unit, etc. are quite different from each other. If only a single type of material is used, it is impossible to maximize the performance of each part, and the whole manufacturing process is complicated and requires a great number of machining procedures, resulting in a low productivity, high energy consumption and severe environmental pollution.
Manufacturing methods for combined camshaft include: welding method, pipe expansion method and mechanical knurling method, etc. All these methods mostly involve decomposing the camshaft into individual cam part, intermediate central shaft part, drive part, etc., and then connecting the various parts to form a complete camshaft by using different manufacturing processes and methods. Of these methods, the mechanical knurling method can be used to connect the cam and the central shaft by making splines. Although the splined connection can ensure a precise cam angle position, it still has the following demerits: firstly, in order to form splines on the inner circumferential surface of the cam and the outer circumferential surface of the central shaft, the structure of the parts is made complicated. In addition, a special-purposed machine must be used to form the splines, and the parts are subject to carburization treatment to ensure a good part performance, resulting in complicated manufacturing process, long time consumption and high cost. Secondly, for the splined connection between the cam and the central shaft, only one cam can be connected on the central shaft at a time, and the assembly of one camshaft can only be completed by multiple sub-assemblies.